In recent years, in wireless communication, and particularly in mobile communication, a variety of information other than speech, for instance images and data, have become subject to transmission. A surge in the future is foreseen in the demand for transmission of a variety of contents, and along with this the need for highly reliable and high-speed transmission is growing. Nevertheless, when high-speed transmission is performed in mobile communication, it is still difficult to ignore the impact of delayed waves caused by multi-path, and the deterioration of transmission characteristics due to frequency selective fading is more than possible.
One variety of techniques to cope with frequency selective fading is multi-carrier modulation schemes such as OFDM modulation schemes. A multi-carrier modulation scheme is a technique whereby data is transmitted on a number of carrier waves (i.e. subcarriers), and the transmission speed is reduced to a degree where no frequency selective fading occurs, thereby enabling high-speed transmission.
According to OFDM modulation schemes, in particular, a number of subcarriers upon which data is allocated are orthogonal to each other, so that the efficiency of frequency use is the highest of all multi-carrier modulation schemes. Moreover, OFDM modulation schemes are implementable through relatively simple configurations. In view of these, OFDM modulation schemes are being discussed in various ways as a measure against frequency selective fading.
Another variety of the techniques to cope with frequency selective fading is spectrum spreading schemes. A spectrum spreading scheme is a technique whereby a signal is spread on a frequency axis using a spreading code called a PN code, thereby, through spreading gain, enhancing the degree of tolerance for interference. Spectrum spreading schemes include direct spreading schemes and frequency hopping schemes. Of these, the use of CDMA (Code Division Multiple Access) schemes that employ direct spreading schemes in IMT-2000, the next generation's mobile communication, has been decided.
Of late, furthermore, OFDM/CDMA schemes that combine these OFDM modulation schemes and CDMA schemes draw attention. These OFDM/CDMA schemes can be roughly classified into time domain spreading schemes and frequency domain spreading schemes. A conventional wireless communication apparatus of an OFDM/CDMA scheme that adopts frequency domain spreading will be described below.
FIG. 1 is a block diagram showing an overview configuration of the transmitting side of a conventional wireless communication apparatus, and FIG. 2 is a block diagram showing an overview configuration of the receiving side of the conventional wireless communication apparatus. Moreover, FIG. 3A shows signal {circle around (1)} in FIG. 1 in a schematic manner; FIG. 3B shows signal {circle around (2)} in FIG. 1 in a schematic manner; and FIG. 3C shows signal {circle around (3)} in FIG. 1 in a schematic manner.
In FIG. 1, N digital symbols (FIG. 3A), which are a serial data sequence, are multiplied in spreading section 10 on a per symbol basis with a spreading code with the spreading factor of M. The chips (FIG. 3B) after the spreading are converted from serial data into parallel data through S/P (Serial/Parallel) conversion section 11, and, in IDFT section 12, N×M symbols are each subjected to inverse Fourier transform processing for in parallel and in sequence. As a result, N×M subcarriers of OFDM symbols are generated (FIG. 3C). In short, with a frequency domain spreading scheme, chips after spreading are each allocated on a frequency axis at discrete times. In other words, chips after spreading are each allocated upon different subcarriers. The N×M subcarriers of OFDM symbols generated in IDFT section 12 are power-amplified in radio transmission section 13, and then released into air through antenna 104.
In FIG. 2, the receiving side of the wireless communication apparatus performs processings that are inverse to the above processings of the transmitting side. That is, a signal received in radio receiving section 21 through antenna 20 is Fourier transformed in DFT section 22, thereby generating N×M chips. The N×M chips generated thus are corrected into a time sequence in P/S (Parallel/Serial) conversion section 23 and thereafter despread in despreading section 24, and N digital symbols, the received data, are obtained.
Meanwhile, conventional, OFDM/CDMA scheme-based wireless communication apparatus have a problem that, when fading is caused by multi-path with small temporal variations in the fading, the state of high correlation between spreading codes continues and makes the likelihood of the occurrence of burst errors high.
In this context, when temporal variations in fading are said to be small, this corresponds to where conditions such as {circle around (1)} and {circle around (2)} in FIG. 4 continue for a long time. In contrast, when temporal variations in fading are said to be large, this corresponds to where conditions such as {circle around (1)} and {circle around (2)} in FIG. 4 switch frequently.
It is commonly known that, when inter-code interference becomes large due to small temporal variations in fading, this condition will continue. When one data has error, it is possible to perform modulation correctly through error correction decoding. However, when a number of errors occur in sequence, the likelihood is very low that error correction decoding makes correct demodulation possible.
As described above, a burst error refers to an error in which data is lost continuously over a relatively long period of time, whereas a random error refers to an error in which data is lost over a relatively short period of time. When a random-type error occurs, performing error correction decoding such as described above makes the likelihood of correct demodulation high, whereas in case of a burst error, this likelihood of correct demodulation is quite low.
Next, the impact of fading due to multi-path will be described in detail.
For instance, assume that there are two spreading codes PN 1 and PN 2 shown below, that spread one bit of data in 4 chips. Here the spreading codes PN 1 and PN 2 are orthogonal to each other.
PN 1: +1, −1, −1, +1
PN 2: −1, +1, −1, +1
In addition, assume that the chips of these spreading codes are allocated upon four subcarriers 1-4 shown in FIG. 5.
Furthermore, assume that a signal transmitted in the arrangement shown in FIG. 5 receives the impact of multi-path fading, and the received signal becomes as shown in FIG. 6.
Rx 1: +0.5, +2, −3, +0.1
Rx 2: +0.5, −2, −3, −0.1
Assume, in other words, that a signal is received in which subcarrier 1 is weighted by 0.5 times, subcarrier 2 is weighted by −2 times, subcarrier 3 is weighted by 3 times, and subcarrier 4 is weighted by 0.1 times. The negative weighting indicates phase inversion.
Assuming that the signal transmitted with the spreading code PN 1 becomes the received signal Rx1, when this signal is despread with the spreading code PN 1, the autocorrelation will be:(+0.5)×(+1)+(+2)×(−1)+(−3)×(−1)+(+0.1)×(+1)=0.5−2+3+0.1=1.6  Equation 1
On the other hand, if the received signal Rx 1 is despread with the spreading code PN 2, the cross correlation element will be as follows:(+0.5)×(−1)+(+2)×(+1)+(−3)×(−1)+(+0.1)×(+1)=−0.5+2+3+0.1=4.6  Equation 2
The resultant cross correlation value from the above second equation is large relative to the resultant autocorrelation value of the above first equation, and so the likelihood is high that a signal transmitted with the spreading code PN 1 is mistaken for a signal transmitted with the spreading code PN 2 and received as such. In case of large temporal variations in fading, the likelihood is high that the condition of fading changes when the next OFDM/CDMA symbols is transmitted, and the relationship between autocorrelation and cross correlation values improves. Still, in case of small temporal variations in fading, the condition prolongs where the impact of fading due to multi-path causes errors, thereby causing burst errors.